This invention relates to an optically addressed optical waveguide sensor system in which a change in the magnitude of the sensed parameter effects a displacement of a component within the sensor system. Examples of such sensors include for instance strain sensors in which the strain is transmitted directly to the sensor system, and pressure sensors in which the pressure effects displacement of a region of some for of diaphragm.
The invention finds particular application in situations requiring remote monitoring of a parameter sensed in some form of hostile environment where it is inadvisable, impractical or inconvenient to employ electrical transducers.
A paper, to which attention is directed, entitled `Fiber-optical relay` by M. Johnson and R. Ulrich, appearing in Applied Physics Letters Volume 31, No. 10 (Nov. 15, 1977), pages 675-6, describes how a steady beam of light ducted down a first optical fibre may be used to excite into mechanical oscillation a short length of a second optical fibre mounted as a cantilever. For this purpose a metallic pad is located close to the fibre core on one side of the second fibre near the root of the cantilever where it will absorb light being guided by that fibre. This absorption produces local heating, and the phenomenon of differential thermal expansion is employed to produce a consequent bending of the fibre. As a result of this bending, the optical coupling efficiency between the two fibres is reduced, and so the heating of the metallic pad is also reduced, with the result that the bending of the fibre is reduced, and thereby the fibre is excited into oscillation by the incident light. The authors of that paper explain that the resonant properties of the device can be exploited for absorption or deposition monitoring. Material absorbed by or deposited upon the cantilever can be expected to provide a loading which will effect its natural frequency of mechanical oscillation. They also explain how the quality factor of the oscillation is affected by the ambient pressure, and suggest that the resonant properties of the device are capable of being exploited for pressure sensing. Another factor mentioned by the authors as a parameter liable to affect this frequency is the thermal expansion of the material of the cantilever, though it is suggested that by an appropriate choice of material this dependence should be capable of being eliminated.